When it is desired to conference a number of telephone lines it is customary to use four-wire operation where the transmit and receive communications signals are on individual wire pairs. The wire pairs from each line are then selectively interconnected through a common amplifier by a switching matrix with the crosspoints of such matrix having the capability of switching all four wires of each line. Such a switching matrix when constructed electromechanically is inherently bulky and when electronics is used to reduce the size the need for four crosspoints per line circuitry becomes prohibitively expensive.
Two-wire conferencing is possible with series and parallel negative impedance, however the negative impedance must be changed for different conference sizes and configurations. Control of the negative impedance can be awkward.
Another problem exists when three or more telephones are connected together in conference fashion with no special conferencing circuitry. In such a situation a loss of signal strength is experienced compared with the signal strength in a typical two telephone connection. As each additional telephone station is added to the conference the signal strength is reduced proportionately. When the crosspoints of the switching matrix have zero resistance, the loss in signal strength is caused by the additional impedance load of each added telephone station.
Thus, a need exists in the art for a circuit which, operating in conjunction with other such circuits, converts a transmission path, in an economical manner, from multiwire operation to bidirectional single-wire operation, without reducing the signal strength as additional circuits are added. Such a circuit would then be useful for, among other things, conferencing large numbers of communication lines without requiring extensive switching network crosspoint matrices.